Catalytic borane-to-borylene conversion and the generality of boron-to-metal alpha-hydride migration in aminoborane activation

The 2011 discovery that aminodihydroboranes $H_2BNR_2$ undergo B–H oxidative addition at $Ir(PMe_3)_3$ to give an isolable primary hydridoboryl complex, which then converts either by intramolecular B→Ir alpha-hydride migration or by external hydride abstraction into cationic aminoborylene complexes, posed two explicit further-study questions: whether this borane-to-borylene pathway generalizes to other metals, other borane substrates and varied ligand environments, and whether such direct dehydrogenative borane-to-borylene conversion steps can be incorporated into catalytic cycles. Later literature (2012–2026) has substantially extended borane-to-borylene conversion to ruthenium, rhodium, platinum, nickel and tungsten with aryl dihydroboranes, catecholborane, pinacolborane and borane-base adducts, but through routes other than B→M alpha-hydride migration (concerted dehydrogenation, B–O cleavage, halide ejection), so the migration step itself remains demonstrated only in the original iridium aminoborane system, and the formation mechanisms of the later systems are explicitly unresolved. No turnover-generating catalytic conversion of boranes into borylene complexes or borylene-derived products has been reported. The open problem is the surviving core: achieving catalytic, turnover-generating borane-to-borylene conversion with demonstrated borylene intermediacy, and explicitly demonstrating B→M alpha-hydride migration in additional metal, ligand and substrate combinations.

Background

Amine-boranes ($H_3B\cdot NR_3$) are Lewis adducts of borane that release hydrogen stepwise; their first dehydrogenation products, aminoboranes $H_2B{=}NR_2$, are formal boron-nitrogen analogues of alkenes and are studied both in hydrogen-storage chemistry and as building blocks for BN-containing materials. A metal boryl complex contains a transition-metal-boron bond to a $BR_2$ ligand; such complexes are typically accessed by oxidative addition of a B–H bond to a low-valent metal, a step that formally splits the bond and transfers H and the boryl fragment to the metal. Oxidative addition of conventional boranes gives secondary boryls, whereas primary hydridoboryl complexes $L_nM\{B(H)R\}$, in which boron still carries a hydrogen substituent, are rare - yet they are precisely the species needed for a second dehydrogenation step. A borylene complex $L_nM{=}BR$ is the boron analogue of a carbene (alkylidene) complex: two-coordinate, formally electron-deficient boron bound to the metal by a multiple bond. Alpha-hydride migration is a 1,2-shift of hydride from the atom directly bound to the metal (the alpha atom) onto the metal itself; for E = C, Si and Ge, alpha-substituent migration at an $M{-}ER_n$ ligand is a textbook route to $M{=}E$ multiple bonds, but before 2011 no such migration with E = B had been demonstrated. The closest precedent was the finding of Alcaraz, Sabo-Etienne and co-workers (JACS 2008) that the aryl dihydroborane $MesBH_2$ ($Mes$ = 2,4,6-trimethylphenyl) dehydrogenates reversibly at $RuHCl(H_2)(PCy_3)_2$ to the terminal borylene $RuHCl(BMes)(PCy_3)_2$ with loss of $H_2$, a result highlighted by Braunschweig and Dewhurst for its implications in hydrogen-storage chemistry; however, no hydridoboryl intermediate was isolated there and no elementary-step mechanism was established. O'Neill, Aldridge and co-workers then showed (JACS 2011) that aminodihydroboranes $H_2BNR_2$ ($R$ = isopropyl, cyclohexyl) oxidatively add across Ir(I) in $Ir(PMe_3)_3Cl$(cyclooctene) to give isolable primary hydridoboryl complexes $Ir(PMe_3)_3(H)Cl\{B(H)NR_2\}$; chloride abstraction with $Na[BAr^F_4]$ ($BAr^F_4$ = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) then triggers intramolecular B→Ir alpha-hydride migration to the cationic borylene dihydride $fac$-$[Ir(PMe_3)_3(H)_2(BNR_2)]^+$, while external hydride abstraction with $[Ph_3C][B(C_6F_5)_4]$ gives the meridional borylene cation $mer$-$[Ir(PMe_3)_3(H)Cl(BN(iPr)_2)]^+$. This sequence provided the first explicit demonstration that a hydride can migrate intramolecularly from boron to a transition metal, completing a stepwise, intermediate-level picture of direct borane-to-borylene conversion - but every step was stoichiometric, and the authors explicitly deferred the questions of how far the pathway generalizes and whether it can be made catalytic.

Problem Statement

The source study left open whether its borane-to-borylene pathway can be generalized and made catalytic. After the partial generality progress recorded above, the surviving open problem has two linked objectives: (1) Catalytic incorporation: construct and demonstrate a turnover-generating cycle in which a borane substrate (an amine-borane, an aminoborane $H_2B{=}NR_2$, or a related dihydroborane $H_2BR$) is converted at a transition metal, with net dehydrogenation, into either an isolable borylene complex released or transferred off the catalyst metal (for example by borylene transfer to a second metal fragment) or a borylene-derived product such as a B-C or B-B coupled species, the borylene formed at the catalyst metal itself acting only as a transient intermediate of that cycle rather than as the isolated product, with $H_2$ (or, where a hydride acceptor drives turnover, an accounted hydride-transfer product formed without consuming the catalyst) as by-product and regeneration of the active metal fragment, together with evidence that a borylene-forming step (B-H oxidative addition to a primary hydridoboryl followed by B-to-metal alpha-hydride migration) lies on the catalytic pathway - for example by intermediate observation, trapping, or isotope labeling that tracks hydride movement from boron to metal or into $H_2$. (2) Mechanistic generality of the alpha-hydride migration step: explicitly demonstrate intramolecular B-to-metal alpha-hydride migration from a primary hydridoboryl ligand in at least one metal, ligand and substrate combination beyond the original $Ir(PMe_3)_3$/aminodihydroborane system - that is, in a system differing from the original in its metal center, its ancillary ligand set, or its borane substrate, and not merely in the choice of N-substituents already examined at iridium - distinguishing it from concerted four-centre reductive elimination, external hydride abstraction, and B-O or B-N cleavage routes. Both objectives must be satisfied; they may be met either by separate experiments or by a single experiment in which the turnover-generating cycle of (1) is run in the non-original system used for (2), so that the demonstration required by (2) also supplies the pathway evidence required by (1). Both objectives address the same scientific question - whether the demonstrated elementary-step sequence is a general and catalytically usable entry into low-valent boron.

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